There are known leak detectors of the kind equipped with a test gas detector (commonly a mass spectrometer), a high vacuum pump in which the test gas flows in the opposite direction, as well as a first relatively small back-up pump downstream with respect to the high vacuum pump. Moreover, a further back-up pump is provided having a relatively high pumping speed which serves the purpose of evacuating the test pieces or a test chamber in which leaks in the test pieces are detected. One reason for the use of two back-up pumps is, that on the one hand the test piece or test chamber shall be evacuated as quickly as possible. For this, the pumping speed of the separate back-up pump is made as high as possible. Production of the back-up vacuum required by the high vacuum pump would be possible with the aid of this second high vacuum pump; however, one would suffer a substantial loss in sensitivity. In order to avoid this, a second back-up pump having a relatively low pumping speed is connected downstream of the high vacuum pump.
It is the task of the present invention to reduce the complexity of generating the back-up vacuum in a leak detector of the kind affected here without having to suffer a loss in sensitivity.